Now scientists have discovered this may be because such a diet affects cholesterol-clearing substances in the brain.

They hope the discovery could lead to new drugs which allow the clogging fats to be cleared more effectively and so help slow down the progression of the debilitating brain condition.

In Britain 500,000 people have Alzheimer's Disease in which the progressive loss of their brain cells leads to memory loss, mood changes and eventually death.

One of the key characteristics of people with the condition is the formation of clumps, or 'plaques' of beta amyloid proteins which are thought to destroy brain cells.

Scientists increasingly believe diet and lifestyle may affect the build up of these damaging proteins.

Studies have found a Mediterranean-style diet rich in plant foods and fish and low in red meat cuts the risk of developing the brain disease by up to two-thirds.

Research in mice has also found that those given high-cholesterol diets have more amyloid beta proteins in their brain.

And there is growing evidence that taking cholesterol-lowering statins makes people less likely to develop Alzheimer's later in life.

To understand what lay behind this trend, Dr Brett Garner, of the Prince of Wales Medical Research Institute in Sydney, Australia, and his colleagues used human and animal cells to probe how brain cells regulate their levels of cholesterol.

In the arteries it is known that ABC proteins help control cholesterol levels by expelling it from the immune cells.

The study, reported in the Journal of Biological Chemistry found these proteins were also present in the brain cells.

When the boosted levels of the proteins by tweaking genes that affect production, cell lines production of amyloid beta protein fell.

The study also identified another protein in brain cells called apoE that regulates cholesterol removal from brain cells.

Dr Garner told New Scientist magazine that drugs that increase expression of these proteins might slow the progression of Alzheimer's.

Similar drugs are already being used for research into heart disease.

He said: "A lot of people think there could be converging factors involved in these diseases."

Large amounts of harmful cholesterol are found in foods high in saturated fats such as red meat, butter, cheese and offal such as liver and kidneys.

If people have a diet high in saturated fats, their liver produces more of the harmful form of cholesterol called LDL, which is linked to a higher risk of heart disease and stroke.

Scientists increasingly believe an unhealthy diet may be a contributing factor in developing dementia.

Previous research has found fish oil capsules may help slow the mental decline of those with very mild Alzheimer's disease.

Last September a team from Vanderbilt University in Tennessee found drinking fruit and vegetable juices more than three times a week could dramatically cut the chances of developing the condition.

Researchers from who followed almost 2,000 volunteers for up to ten years found the risk of Alzheimer's was 76 per cent lower for those who drank juices more than three times a week compared with those who drank them less than once a week.

Japanese scientists also found last year that green tea could halve the risk of mental decline in old age.

They found those who drank the tea the most - more than two cups a day - had a 54 per cent lower risk of dementia than those who drank the least.